The present invention relates to the digital imaging art. It finds particular application in conjunction with a method and apparatus for rendering a continuous tone (a.k.a. contone) image representation with multiple digital halftoning techniques, and will be described with particular reference thereto.
In the digital reproduction of documents, an image is conveniently represented as a bitmap, which may be described as an electronic image with discrete signals (hereinafter, pixels) defined by position and density. In such a system, density is described as one level in a number of possible states or levels. When more than two levels of density are used in the description of the image, the levels are often termed “gray”, indicating that they vary between a maximum and minimum, and without reference to their actual color.
Most printing systems have the ability to reproduce an image with a small number of levels, most commonly two, although other numbers are possible. Common input devices including document scanners, digital cameras and the computer imagery generators, however, are capable of describing an image with a substantially larger number of gray levels, with 256 levels a commonly selected number, although larger and smaller levels are possible. It is required that an image initially described at a large set of levels also be describable at a smaller set of levels, in a manner which captures the intent of the user.
For color images, a plurality of bitmaps, each forming a color separation are combined. Each color separation may be defined by a number of gray levels in excess of the capability of the printer. In digital reproduction of color documents, each of the color separations is reduced from the input number of levels to a smaller output number of levels. The multiple color separations are combined together at printing to yield the final color print. Commonly, color documents are formed using cyan, magenta and yellow colorants or cyan, magenta, yellow and black colorants. A larger number or alternative colorants may also be used.
In printing digital image representations of a contone image, the desired density over an area is commonly achieved by halftoning, where image density variation is represented by placing greater or less numbers of ON pixels in a discrete area of the image. One digital halftoning technique for reproducing a contone image representation is error diffusion. Error diffusion attempts to maintain gray by making the conversion from gray pixels to binary or other level pixels on a pixel-by-pixel basis. That is, each pixel is examined with respect to a threshold, and the difference between the gray level pixel value and the output value is forwarded to a selected group or set of neighboring pixels, in accordance with a weighting scheme. Error diffusion usually produces halftones with decent overall image quality. However, it also introduces artifacts such as “worms” and periodic patterns. The worm artifacts depend on input values, and generally occur at the highlight and shadow regions. The periodic patterns are most severe when the input values happen to be a simple fraction of the output range, e.g. ½×255, ⅓×255, ⅔×255, etc.
Another digital halftoning technique for reproducing a contone image representation is stochastic screening. In U.S. Pat. No. 5,673,121 by Shen-Ge Wang (a co-inventor of the present application), an idealized stochastic screen is characterized by having all of the predominant color dots (black or white) uniformly distributed. A process is described to approach this optimization by iteratively selecting pairs of threshold levels in the screen matrix, and measuring the approach to the idealized stochastic screen. The threshold values are then swapped in position to determine whether the swap improves the measurement or not. If it does, the swap is maintained. The process is iterated until the desired result is obtained. Stochastic screening typically creates excellent halftones at the highlights and shadows. It can also avoid periodic patterns. However, stochastic screened halftones have relatively higher overall noise levels and lack the inherent edge enhancement typically provided by error diffusion.
Accordingly, it is considered desirable to develop a new and improved method and apparatus for rendering a continuous tone image representation with multiple digital halftoning techniques, that meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.